I took the following quote from Joanna Zorya at Martial Tai Chi:
Our own teaching completely rejects the concept of qi, also known as chi, ch'i or ki. Other instructors coming to the MTA [Martial Tai Chi] should also reject the idea completely. However, on this website there are a couple of articles which specifically deal with the issue. Qi is also dealt with briefly on our "Taiji Concepts" DVD - the clip (in "3 internal harmonies excerpts") is shown on our "Techniques" video clips page. We have found it necessary to address the issue of qi, because most people in the Tai Chi mainstream are utterly obsessed with it, and we wanted to make our position on it absolutely clear. The concept is at best obsolete and at worst dangerous. Significantly, the notion of qi is simply not true.
The way I see it the word Qi is polysemous. It has many different meanings depending on context. So if the teachers at Martial Tai Chi want to ban the word, they aren't necessarily banning the concepts that come with it.
The word qi might in a particular context mean the totality of everything you can feel. Or it might mean the feeling of blood or lymph pulsing through your body. But in another context it means the bubbles in a glass of soda pop. If the term qi is not clearly defined in context, it can be used to create intensional vagueness. Such vagueness is often used by Charismatics to create a feeling of authority among witnesses to a performance of healing or other subordinating demonstrations of power.
However, we really aren't sure what it is we are feeling inside and even outside our bodies. The term qi can be used in conjunction with other words to communicate the density, directionality, size, or relative temperature of something we feel. I'm not going to argue that it is a necessity, just that it can be used appropriately.
Here is my LONG definition of Qi. I wrote this 10 years ago, so it may need some updating, but perhaps readers will have suggestions.
UPDATE: I've had some interesting exchanges with Joanna Zorya in the comments for a previous post on Lineage. I mention these books: